A number of potential advantages have led the automotive industry to look with increasing interest toward utilizing common rail (manifold) high pressure direct injection for gasoline engines. A number of design constraints or difficulties seem to stand in the way of fully achieving the advantages.
The pressurization of fuel to high levels (e.g., above 100 bar) requires considerable pumping power, which generates considerable heat. Moreover, the industry is looking for even higher rail pressures, above 200 bar. This heat could be dissipated to a large extent, if all the fuel that is pressurized, can be quickly injected into the engine cylinders.
This is not possible, however, because the fuel pump flow rate is typically sized for engine cranking, which may be at 20-30 bar pressure at a high quantity discharge flow rate, whereas typical steady state cruising conditions require much lower quantity flow rates at 100 bar. Therefore, in a conventional pumping scheme, the volume of fuel raised to injection pressure during the course of an hour of typical vehicle use, is much greater than the volume of fuel actually injected during that same hour of use. Although pre-metering and various spill control techniques can be used to some advantage in this regard, none of these techniques satisfactorily regulates the power output of the high pressure pump itself.
Another difficulty is encountered with high pressure pumps that are driven directly by the engine (e.g., crank shaft, cam shaft, accessory belt). During transients when fuel demand is low (e.g., downhill or during gear shifting), the engine continues to turn and the, pump continues to deliver high pressure fuel to the common rail that may already be at maximum pressure.